The vegetation of the Rock of Gibraltar is, on the whole, typical Mediterranean in nature. This consists largely of dense scrub of the type known as maquis, but there are other areas of more open, lower scrub known as garigue. However due to the geologic and geographic nature of the Rock there is considerate variation in the type of scrub, and also in the variety of habitats available to plants. Apart from scrub we have extensive sea cliffs, a limited but important rocky shoreline, the unique great sand slopes of the east side, rocky limestone outcrops and fissures, the steppe conditions of Windmill Hill, and the remains of the sandy isthmus which linked Gibraltar to Spain found at North Front Cemetery. Gibraltar is a limestone mountain and so the soil is alkaline. In contrasts the mountains in the surrounding area of Spain are largely sandstone and so the soil is more acid. As a result of this there are many species found on the Rock which are rare or not found in the surrounding region. At the same time, the geology of the Rock is similar to that of North Africa, and so there are species common to both regions. All these factors provide a variety of habitats which give rise to an extensive flora, including some plants which are unique to the Rock. The flora of Gibraltar consists of some 530 species, representing almost 90 families and 330 genera. The majority of these are native species, but some of the 530 are species which have been introduced and become naturalised. The principal flowering months are March to May, but flowers may be seen virtually all the year round. However, very few will be out in the summer months (July to August) when very little if any rain falls. Shortly after the first rains in September, new growth quickly becomes evident, and by November, a large number of plants are already visible.